In deep drillings for developing oil or natural gas deposits, the use of drilling muds and cement slurries has been known for a long time. The purpose of drilling muds is to deliver the rock fragments obtained by drilling and the so-called drilling fines to the surface, lubricate the bit and the drill string, to seal porous rock layers and to compensate the deposit pressure by means of hydrostatic pressure. For the last-mentioned reason, drilling muds must have increased specific weight. This is achieved by adding, preferably, heavy spar, salts or clay. Further important features of drilling muds are temperature resistance and suitable flow properties which are only little affected by changes in the electrolyte concentration. The most widespread additives for controlling viscosity and loss of water of drilling muds are polymers, such as starch, carboxymethylcellulose and carboxymethylhydroxyethylcellulose. Since the 1950s, copolymers of the acrylamide/acrylate type have been used predominantly in salt-free drilling mud systems. During the 1970s, salt-stable copolymers with monomers (U.S. Pat. Nos. 3,629,101, 4,048,077, 4,309,523) containing sulfo groups and being stable up to more than 200.degree. C. were developed.
Furthermore, cement slurries and completion fluids are used as drilling fluids in deep drillings for oil or natural gas. After they have reached a certain depth, iron pipes, so-called casings, are introduced into the borehole, through the cavity of which the bit is passed for drilling of the next lower rock layers. To this end, the casings have to be fixed, i.e. a cement slurry has to be pumped into the cavity between the rock and the outer walls of the casings, the so-called annular space, which slurry hardens to give solid stone. The resulting cement stone has to be impermeable to gases and liquids, so that no gas and/or oil can flow from the reservoir formation into other formations or to the surface. Very high demands are made on the cement slurry to be pumped. It should be readily pumpable, i.e. have the lowest possible viscosity, but nevertheless show no separation. The water release of the cement slurry to the porous rock should be low so as to prevent formation of thick filtercakes on the wall of the borehole, which would increase the pumping pressure as a result of the narrowing in annular space to such an extent that the porous rock forms cracks. Moreover, in the case of excessive water release, the cement slurry would not completely set and would become permeable to gas and oil. On the other hand, the resulting cement jacket in the annular space must quickly reach certain strengths, and during setting no shrinkage must occur leading to flow channels for gas, oil and water. Optimum adjustment of the properties of the cement slurry is only possible by means of additives.
The most important additives for controlling setting are retardants, accelerators, dispersants for liquefaction and agents for reducing water loss. Some of these additives have more than one function. Dispersants such as lignosulfonates and polymethylenenapthalenesulfonates retard setting and reduce water loss to some extent. Some agents for reducing water loss retard setting and drastically increase viscosity.
Effective agents for reducing water loss from cement and gypsum slurries used in practice comprise a wide range of polymers, copolymers and combinations thereof. The first effective products, which are still used today, are hydroxyethyl- and carboxymethylhydroxyethylcellulose. Hydroxyethylcellulose increases viscosity and somewhat retards setting. Carboxymethylhydroxyethylcellulose has a stronger retarding effect, which, however, can be compensated by accelerators. With increasing temperature, the effect of the cellulose ethers drops substantially. In subsequent years, many different fully synthetic polymers of higher temperature stability have been proposed and are in use. U.S. Pat. No. 3,994,852 describes polyvinylpyrrolidone/polyacrylamide copolymers, U.S. Pat. No. 4,015,991 describes hydrolysed acrylamide/2-acrylamido 2-methylpropanesulfonate copolymers, EP 0,192,447 describes dimethylacrylamide/2-acrylamido-2-methylpropanesulfonate copolymers and EP 0,116,671 describes 2-acrylamido-2-methylpropanesulfonate, acrylamide (partially hydrolysed) and vinylamide terpolymers, which are used in cement slurries for controlling water loss.
It is necessary to adjust the cement slurry in each case using the cement available at the derrick and the additives in accordance with requirements.
The large number of compounds developed makes it clear that it is always difficult to formulate an optimum cement slurry. In the case where the individual parameters are predetermined by the type of cementation, the necessary properties have to be set to acceptable values using additives. The high number of compounds developed for reducing water loss shows how problematical it is in most cases to set the water release to the required value without significantly increasing viscosity, to set the setting time to the required value and to minimize sedimentation. The previously known polymers reducing water loss strongly increase the viscosity of the cement slurries, which in most cases have high density, to greater or lesser degrees. However, for ready pumpability of the cement slurries, viscosity must be kept low. The pumping rate should be such that a turbulent flow is possible. Only under these conditions does complete displacement of the drilling mud take place. This is a prerequisite of good cementation. In the case of inclined drillings, the mud can only be readily displaced by a strong turbulent flow.
For completing oil and natural gas wells, salt solutions of high density are used, which compensate the deposit pressure. During this, their infiltration of the deposit must be kept to a minimum. However, hydroxyethylcelluloses are not suitable for the temperatures of more than 200.degree. C. occurring there and the high salinities and densities brought about by CaCl.sub.2 and CaBr.sub.2.